Synopsis

Noise pollution can significantly affect sound processing, even without causing hearing loss. In this study, adult rat subjects are exposed to 100 dB sound level noise for 15 minutes. Functional magnetic resonance imaging (fMRI) and auditory brainstem response (ABR) testing with sound stimulation are performed before, 7 days after, and 14 days after exposure. ABR results show no significant threshold elevation, indicating no significant abnormalities in and near the ear. fMRI results show significant response reductions in the auditory midbrain 7 days after exposure. This suggests transient changes in central auditory gain following acute noise exposure.

Purpose

Methods

Subjects were prepared for fMRI sessions similar to in our previous studies^4-11. Ten normal female Sprague–Dawley rats were employed. Subjects were anesthetized with isoflurane and mechanically ventilated. Warm water was circulated underneath the subject and its vital signs were monitored (SA Instruments).

The acute noise exposure was an octave band noise (8 – 16 kHz) at 100 dB sound pressure level (SPL) lasting for 15 minutes. During exposure, the subject was unrestrained within a cage. The cage was placed below the speaker (MF1, TDT). The speaker output was calibrated in the center of the cage using a microphone (M50, Earthworks) and recorder (FR2, Fostex). After exposure, subjects were placed in standard acoustic conditions in the holding room.

Acoustic stimulation during fMRI was produced by a MF1 speaker and delivered to the ear through a custom sound tube. Pulsed tonal stimuli at 12 and 20 kHz were produced with 100% modulation depth and 50% duty cycle. The sounds were calibrated at the ear end of the tube using the M50 microphone. The stimuli were presented in a block design paradigm as in earlier studies^4,8-12.

fMRI was performed before, 7 days after, and 14 days after exposure. Imaging was performed at 7 T (Pharmascan, Bruker Biospin). The fMRI sequence was: single-shot GE-EPI, FOV = 32×32 mm², matrix = 64×64, slices = 9, slice thickness = 1.2 mm (0.2 mm gap), angle = 56°, TE = 20 ms, and TR = 1000 ms. The third slice was centered on the inferior colliculus (IC). After each time point, subjects were returned to the holding room.

Auditory brainstem responses (ABRs) were recorded at each time point. Subjects were anesthetized by sodium pentobarbital. Subdermal needle electrodes were inserted at the vertex (positive), below the pinna (negative), and at the back (ground). Click stimuli (0.1 ms) were transmitted to the recording ear. ABRs were averaged across 1024 clicks per SPL. SPL was reduced by increasing speaker–ear separation.

fMRI images were processed using standard image registration and normalization algorithms implemented in SPM (Wellcome Trust Centre). T-value maps were also computed using the general linear model implemented in SPM. Statistical comparisons were performed with repeated measures analysis of variance and Tukey’s honest significant difference test. A p-value threshold of 0.05 was defined as significantly different.

Results

Short duration, high SPL acute noise exposure reduces fMRI responses in the IC of the midbrain 7 days after exposure. This is observed in the t-value maps in Fig. 1A, acquired before (0d), 7 days after (7d), and 14 days after (14d) exposure. By 14d, the responses recover to near pre-exposure levels. The t-value difference maps in Fig. 1B further illustrate this transient reduction.

At 7d, fMRI signals from the IC during 12 kHz stimulation are 0.4% lower than 0d (Fig. 2). By 14d, signals are only 0.1% lower than 0d. During 20 kHz stimulation, signals from 7d are 0.2% lower than 0d and signals from 14d are slightly higher than 0d. Signals from 7d are statistically significantly lower than those from 0d and 14d (Fig. 3). The signals from 0d and 14d are statistically similar. In the lateral lemniscus (LL) of the midbrain, signals at 7d are significantly lower than 0d. Acute noise exposure leads to transient reductions in midbrain responses with timescale of weeks.

ABRs from two subjects in Fig. 4 show the lowest SPL at which clearly discernible responses are observed with our system is 11 dB at all time points. This shows that the acute exposure does not induce significant hearing threshold shifts. Therefore, the above fMRI observations likely reflect changes near the midbrain level.

Discussion and Conclusion

Acknowledgements

References

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